HURST CORPORATION STAINLESS STEEL

Transcription

1 STAINLESS HURST CORPORATION STEEL PRIMER www

2 OVERVIEW Stainless steel is an iron-based metal which contains very low carbon levels (compared to mild steel) and various levels of chromium. Chromium combines with oxygen to form an adherent surface film that resists oxidation. There are over 100 kinds of stainless steel, but only five are generally popular. Each type varies in hardness and strength and in corrosion and heat resistance. Type 304 stainless is by far the most popular. It is basically composed of 18% chromium and 8% nickel (sometimes referred to as 18-8 ). Variations of the 18-8, 300 series of stainless steel include types #302, 304L and type #305. The 18-8 stainless steel family represents an excellent combination of corrosion resistance, fabricability, and value, accounting for one half of all U.S. steel production. Type #304 stainless steel especially lends itself to machinability, fabrication and welding. Type #304, with a Brinell hardness of 201 and a Rockwell B scale hardness of 92, is highly impact and corrosion resistant when used within a normal temperature range. Chemical Composition of Type #304 Stainless Steel Element Percentage by Weight Carbon 0.08 Manganese 2.00 Phosphorus Sulfur Chromium Nickel 8.00 Nitrogen 0.10

3 HARDNESS Hardness is a property of a material which quantifies its resistance to plastic deformation, an indicator of the strength of that material. Hardness is measured in one of several different hardness scales, such as Vickers, Brinell, Knoop and Rockwell. The Brinell and Rockwell B scales are generally used to measure hardness in stainless steel. (Note: the Rockwell A scale is used for softer materials; Rockwell C for the hardest materials) Actual hardness testing consists of a precision tip (or ball) that enters the material surface under the force of a specific load. The distance the point moves into the material is associated with the appropriate hardness scale. The deeper the penetration, the softer the material. Although both Brinell and Rockwell hardness testing provide good general hardness information, both internal and external hardnesses may vary. Hardness is directly related to resistance to scratching. If abrasion is an issue, Chromium-Nickel-Molybdenum stainless steels are the appropriate choice. This is particularly true in the pharmaceutical and food industries where stainless steel trays are frequently used to store glass containers. Material Hardness Chart Material Brinell Hardness Rockwell A Scale Rockwell B scale Rockwell C scale # na 92 Below 14 #316L 217 na Glass na 86 na 68/72

4 FINISHES The primary starting stainless steel finish is designated as a Number 2B (standard mill) Finish. Stainless is first cold rolled, and then, annealed and descaled. Lastly a light temper pass is applied after the final annealing on a cold mill with polished rolls. The result is a bright extremely smooth slightly milky finish which lends itself to further polishing. WELDING Normally seams are TIG (fusion) welded, with outside welds ground to a #4 finish. Inside welds are electro-polished. Tabbed corners may be tack welded for increased economy. PERFORATING Custom-pattern hole punching/notching is available to meet customer requirements. Standard punch sizes range from.125 diameter to.50 diameter in 50/50 and 40/60 hole combinations. GAUGE SIZE AND WEIGHT Gauge Thickness Weight per sq. ft lbs lbs lbs lbs lbs

5 Chemical composition of Type #316 Stainless Steel Element L 316H Carbon Manganese Silicone Phosphorus Sulfur Chromium Molybdenum Nickel Nitrogen SELECTION 304 stainless steel is the best all-around choice for general wear and corrosion resistance. 316 stainless steel contains an addition of molybdenum that gives it improved corrosion resistance. This is particularly apparent for pitting and crevice corrosion in chloride environments. 316L, the low carbon version of 316 stainless steel, is immune to grain boundary carbide precipitation (sensitisation). This makes it suited to use in heavy gauge (over about 6mm) welded components. 316H stainless steel, the high carbon variant, should be employed for elevated temperature applications over 500. (316Ti with.5% titanium is best for applications over 800 )

6 PICKLING Pickling, or chemical descaling, is performed to remove tightly adherent oxide films resulting from hot-forming, heat treating, welding and other high temperature operations. Welding often produces complex oxides that can vary in color. All of these oxides are referred to as scale and must be removed. Generally a descaling solution of nitric and hydrofluoric acids is used. Mechanical alternatives such as sand blasting or wheel abrading may also be performed. PASSIVATING Passivation is a chemical process performed on steels with over an 11% chromium content, to expose the natural invisible (passive), self-repairing chromium oxide film layer upon their surface. It is this passive layer that gives stainless steels their corrosion resistance. If a stainless steel surface is scratched, more chromium is exposed which reacts with oxygen replenishing the chromium oxide passive layer and restoring maximum corrosion resistance. Passivation is necessary because during metal manufacturing and processing functions, particles of iron, tool steel, or abrasives may be imbedded in or smeared on the stainless steel s surface. If allowed to remain, these particles may corrode and produce rust spots on the stainless steel. This is due to the formation of a galvanic couple between two dissimilar metals that can promote a corrosive reaction. Passivation dissolves imbedded and smeared ions and restores the original corrosionresistant stainless steel surface.

7 ELECTROPOLISHING Electro polishing streamlines the microscopic surface of a metal object by removing metal from the object s surface through an electromechanical process similar to, but the opposite of, electroplating (Where surface is added to a metal). In electro polishing, metal is removed ion by ion from the surface of the metal being polished. Electrochemistry replaces traditional metal finishing techniques including grinding, blasting and buffeting. The metal to be polished is immersed into an electrolyte and subjected to a direct electrical current. The object is maintained anodic, with the cathode connection being made to a nearby metal conductor. Smoothness of the metal surface is one of the primary and most advantageous effects of electro polishing. During the process, a film of varying thickness covers the surface of the metal. The film is thickest over micro-depressions and thinnest over microprojections. Because electrical resistance is at a minimum wherever the film is thinnest, electro polishing selectively removes high points (peaks) faster than the rate of attack on the corresponding micro-depressions (valleys) leaving an ultra smooth, clean and bright microscopically featureless surface. Electro polishing benefits include: a. Less friction and drag surface b. Decreases fouling, dirt, and product build-up c. Maximum tarnish resistance d. Maximum heat and chemical corrosion resistance e. Decreased cleaning time f. Improved sterilization and surface maintenance